This invention relates to apparatus for detecting fires, intruders or other abnormal conditions to be monitored within a space under surveillance and for providing alarm actuating signals in response thereto, and, more particularly, to such apparatus of the type employing a plurality of electrically variable sensors which respond to infrared, light or other radiations received thereby. More specifically, the invention is concerned with providing improved detection and alarm apparatus which overcomes the disadvantages and operational limitations of prior devices for the same general purpose, especially with respect to enhanced sensitivity for the conditions actually to be monitored and increased reliability in practical application, including the ability to discriminate against and avoid generating "false alarms" in response to insignificant events such as slow normal changes in ambient conditions.
Known prior detection and alarm systems based upon radiation sensing have historically suffered from either relative insensitivity to the types of conditions actually to be detected or relative vulnerability to "false alarms" caused by insignificant changes in ambient conditions when an effort is made to improve sensitivity, or from the operational limitations imposed by trying to compromise those conflicting considerations in a manner sacrificing optimality as to both. The earliest such systems, which are still in common use despite their disadvantages, employed a single radiation sensitive sensor whose internal electrical impedance characteristic (usually the series resistance therethrough, although the amount of electrical current produced by self-generating types radiation sensing components may have been also equivalently employed) is altered in response to the level of radiation applied to the sensor. In such early systems, the single sensor has been employed in some suitable electrical circuit whose output is essentially controlled in correlation with the variable impedance of the single sensor and, therefore, the variable level of radiations to which such sensor is subjected from the space being monitored by it; typically, such single sensor has been employed as a series impedance in a single arm of a bridge circuit whose other arms contain fixed lumped impedance elements. Thus, it will be appreciated that the sensitivity of such earlier systems has been inherently limited in the first instance in two ways -- first, by virtue of the employment of only a single sensor to sense radiations on a gross basis for the entire space being monitored, which is thus inherently responsive only to changes in the overall level of radiations received from the space, and secondly, by virtue of the relative insensitivity of bridge circuits having a variable impedance (the single sensor) in only a single arm thereof to respond to quick small changes. It will be appreciated that even efforts to focus radiations from different parts of a space upon a single sensor (as by multiple mirrors) is alone inherently incapable of overcoming the inevitable limitation upon sensitivity imposed by the electrical aspects of employing only a single sensor. Moreover, attempts to improve sensitivity characteristics in such single sensor systems by providing sufficient "follow-up" amplification have resulted in loss of reliability through increased vulnerability to "false alarms".
It is understood that some attempts have been made to improve the characteristics of the early single sensor systems by employing a plurality of sensors, redundantly responsive on an essentially gross basis to radiations from the same space, electrically coupled in series or parallel with each other, so that an additive electrical effect may be derived with a given type of sensing components from a given amount of radiation from the space under surveillance acting upon multiple sensors; but such attempts are further understood to have involved merely treating the intercoupled plurality of sensors as essentially a single variable impedance element with regard to the coupling thereof into a single arm of a bridge circuit or the like for purposes of providing an electrical output in which changes could be detected. This approach also failed to solve, or apparently even to grasp the real nature and causes of, the problems inherent in all known earlier systems, which arise from their treating the space under surveillance on a gross basis and from their employment of what effectively amounts to only a single variable impedance sensing means in only a single arm of a bridge or some other circuitry arrangement in which variations in the sensing responsive impedance can only "work against" fixed impedance elements in some voltage dividing arrangement to produce variations in an electrical output representative of the changes in radiation conditions to be sensed.
It is also understood that efforts have been made to decrease the vulnerability of the early single sensor systems to "false alarms" by attempts to improve the signal-to-noise ratio of the system involving the employment of sensor type components in more than one arm of a bridge circuit, but with only a single one of such components in a single arm of the bridge circuit being exposed and responsive to radiations from the space being monitored, and the remainder of such components being shielded from such radiations and effectively employed merely as lumped impedance elements whose essentially fixed values are automatically "adjusted" momentarily in response to electrical "noise" factors or ambient changes affecting the impedances of all such components (but unrelated to significant radiation parameters of the space being monitored). Although such arrangements can increase the general stability of a system by tending to offset the effects of such "internal" factors as the temperature-impedance coefficients of the components, supply voltage variations or the like, it will be clear that they have not solved, or even directly addressed, the problem of the limited sensitivity inherent in the employment of only a single monitored radiation sensing sensor and the susceptibility of such radiation sensing systems to "false alarms".